Interference suppressor for radio receivers



July 29, 1941. H. o. PETERSON ETAL 2,250,550

INTERFERENCE SUPPRESSOR FOR RADIO RECEIVERS Filed NOV. 29, 1938 NARROWBAA/0 PASS FILTER RECTIFIER same; 0% TONE MODULA nous ass/1mm MIXER AND1.1-. AMPLIFIER INVENTORS HAROLD OLAF PETERSON R. LEE H0 INGSWORTHATTORNEY AMPL/F/ER Patented July 29, 1941 UNETED STATES mrnarnmzncnsnrrnnssoa non name RECEIVERS Application November 29, 1938, Serial No.242,878 4 Claims. (01. 250-8) This invention relates to an interferencesuppressor for a radio receiver. More particularly our invention isintended for use in radio telegraphic reception for the purpose oflimiting the voltage of the signalling energy as applied to thereceiving devices, applying tone modulations to the interrupted carrierwave signals, and filtering out the product of the tone modulator mixerthrough a. suitable band-pass filter having the desired characteristicsfor reducing certain types of atmospheric static and man-made static.

It is an object of our invention to provide a circuit arrangement whichrenders a radio receiver relatively free from interference such asatmospheric and man-made static.

Another object of our invention is to provide a system including avoltage limiter, a balanced modulator, and a narrow band filter whichare so co-ordinated as to obtain satisfactory signal reception whilegreatly attenuating the effects of noise-clicks and other shortimpulses.

A still further object of our invention is to improve the conditions forradio telegraph signal reception so that mutilation of the signals shallbe reduced to a minimum.

The foregoing and other objects of our invention, together with a fulldescription of the means and methods to be adopted in carrying out theseobjects, are set forth in the following specification, reference beingmade to the accompanying drawing in which Figure 1 shows a preferredcircuit arrangement, and

Fig. 2 shows a group of related graphs by which the theory of operationis more fully explained.

Referring to Fig. 1, we show diagrammatically therein the essentialapparatus for carrying out the invention. The telegraph signals arereceived on an antenna I of any suitable type and applied to a radiofrequency amplifier 2. If a heterodyne receiver is to be employed, then,by means of a local oscillator 2| and a mixer circuit (not shown) whichmay be understood to be included in the apparatus group 3, anintermediate frequency wave may be derived and after amplificationapplied to the primary winding of a transformer T, the secondary ofwhich is in circuit with the rectifier tube 4 and on the other side witha resistor 5 leading to ground. A rectified current which flows throughthe resistor 5 is used to control a voltage limiter tube 1 whichpreferably comprises a cathode 22, a control grid 23, a screen grid 24and an anode 25. The output circuit of this voltage limiter tube may betraced from the grounded cathode through a retive state the potential ofthe negative terminal of the source 8 with respect to the cathode issuch as toblock two amplifier tubes II and 12 which are disposed in apush-pull relationship to one another. The control grid circuit forthese tubes includes the secondary winding 26 of a transformer 2?, thissecondary 26 having a mid-'- tap which is connected through a biasingsource H] with the negative terminal of the anode cur rent source 8 forthe tubel.

The transformer 21 hasa primary which may, if desired, be constituted byone or two separate windings. Two windings are shown in the diagram byway of example and these are connected in parallel to a source of tonemodulations (not shown). The output circuit of the tubes l l and l 2includes the center-tapped primary winding 28 of an output transformer29. The secondary 30 of this transformer delivers an interrupted tonemodulated signal. to a narrow band pass filter I l, the-amplitude ofthis signal being controlled by a potentiometer I3. The output energyfrom the filter I4 is preferablyrectified by a device it, the directcurrent terminals [5 of which may be connectedto any indicating orrecording device. 1 1 In-th operation of the circuit arrangement of Fig.1 it will be seen that whenever signal or noise energy is collected bythe antenna l a rectified current is caused to flow through the resistor5. which current blocks the tube '1. The consequent reduction of theplate current of this tube to zero places a normal bias on the controlgrids of the tubes I l and I2.

sultant operation of the transformer 29 is such as to apply themodulations to the filter l4; When, however, a no-signal conditionexists the current conducted through resistor 9 and the tube 1 is suchas to bias the grids of the tubes II and I2 sufficiently to block thesetubes. 'Hence no tone modulations are applied to the filter I4.

Four wave graphs A, B, C and D are shown in Fig. 2. Graph A representstypically the time plot of a dot, a dash and a short click of highamplitude. Curve B shows the product of the voltage limiter whichreflects the dots, dashes and interference energy at acommon amplitudelevel. Curve C shows the resultant of the tone modulated Waves asapplied through the push- The tone frequency modulations are, therefore,impressed upon thepushpull output circuit of these tubes and the repullamplifier under control of the voltage limiter. Zero current prevailswhenever the tubes H and I2 are biased to cut-off. Graph D shows therectifier output from the unit [6 as impressed on the terminals l5 inconsequence of the tone modulated input energy applied to that filter.It will be particularly noted that the rectified signal as fed to theutilization circuit shows a gradual rise of signalling energy from azero point so that interference waves of very short duration do not havetime to build up to the amplitude level of the dot and dash signals.This is a characteristic of the operation of the narrow band filtersince it does not respond to sudden changes. A comparison of graph Dwith graph A shows, therefore, how effectively the static impulses andother interference waves of short duration are suppressed. In fact, thefilter barely responds to such impulses and they do not cause therecorder to make any appreciable response.

It will be understood by those skilled in the art that our invention issusceptible to various modifications without departing from the spiritthereof. The scope of the invention is, therefore, a

limited only in accordance with the claims.

We claim: 1. In a radio telegraph signal receiving system,

the method of suppressing the effects of short bursts of static andother momentary interference phenomena which accompany the reception ofuseful radiant signal energy, which method comprises heterodyning saidradiant energy with local oscillations to produce an intermediatefrequency wave, rectifying said wave, producing a substantially squaremark-and-space signal wave which is a limited derivative of therectified wave, locally generating a tone frequency potential, keyingsaid potential under control of said square wave, and filtering thekeyed tone frequency potential in such manner as to suppress an initialcycle thereof which occurs at the front edge of a marking element ofsaid signal wave, andalso to suppress the. effects of a momentaryburstof static received during. a spacing element of said signal wave.

2. In a radio telegraph signal receiver, an electronic voltage limiteroperable by incoming signals, a balanced circuit arrangement comprisingtwo discharge tubes having input electrodes connected to oppositesecondary terminals of an input transformer and anodes connected toopposite primary terminals of an output transformer,

a connection from the output side of said voltage limiter to a mid-tapon the secondary of said input transformer, a source of direct currentnegatively connected to the cathodes of said discharge tubes andpositively connected to the anodes thereof through a mid-tap on theprimary of said output transformer, a tone-frequency modulator indriving connection with a primary on said input transformer, and anarrow bandpass filter having input terminals in driven connection witha secondary on said output transformer.

3. In a radio telegraph signal receiver electronic means for limitingthe efiects of incoming signalling and interference energy, a push-pulltone modulator circuit comprising a pair of discharge tubes havinggrid-biasing means symmetrically connected thereto, meansinterconnecting the limiting means and the grid-biasing means in suchmanner as to simultaneously shift the control bias of said pair of tubesfrom a cutoff point to a working point, and vice versa, in response tothe operation of said limiting means, and a narrow band-pass filter fedwith energy from said tone modulator, said filter being char acterizedin that it is relatively insensitive to an initial cycle of outputenergy from said tone modulator.

4. Apparatus for suppressing the effects of transient bursts of staticand other momentary interference phenomena comprising, a radiant energycollector andamplifier, a push-pull discharge tube stage under controlof signaling output energy from said amplifier, a voltage limiterinterposed between said amplifier and said pushpull stage, means forlocally generating tone modulations, means for feeding said tonemodulations to said push-pull stage while simultaneously causing saidmodulations to be keyed on and off by said limiter in accordance withmarking and spacing intervals of said signaling energy, and a narrowband-pass filter connected to the output side of said push-pull stageand having a time constant such that the effects of said transientbursts of static are substantially suppressed.

HAROLD OLAF PETERSON.

R. LEE HOLLINGSWORTH.

